Electrostatic image fixing method employing ink and record medium having chemical similarity



Dec.'20, 1966 P. A. sTowELL. 3,293,059

' ELECTROSTATIC IMAGE FIXING METHOD EMPLOYING INK AND BY "64 4e/L6 AGENT1 E TOR PHlLIP A. STOWELL P. A. STOWELL Dec. 20, 1966 ELECTROSTATIC'IMAGE FIXING METHOD EMPLOYING INK AND RECORD MEDIUM HAVING CHEMICALSIMILARITY Filed Nov. 29, 1961 2 Sheets-Sheet 2 FIGB FIGS

FI G7 INVENTOR.

PHlLIP A. STOWELL FIGB AGENT United States Patent O M The presentinventior'rbroadly relates to electrostatic printing and moreparticularly,A although not necessarily exclusively, to a novel methodand/or process for fixing electrostatically formed images onto recordingmaterial without the use of external applied heat. With still moreparticularity,fthe invention has to do with a method or processemploying an electrostatic printingink of a chemical formulation orgroup whichvdemonstrates a" natural chemical and mechanical afiinity fora dielectric material bonded to or dispersed throughout' a record mediumor substrate, such for example; an an electrostatic recording paper.'

It is an object of the present invention to providea method of fixingelectrostatically formed images onto electrostatie record materialywithout the use of externally applied heat.

It is another important object of the invention to provide a method'offixing electros-tatically formed images onto electrostatic printingpaper by' means of pressure so as tocold weld or bondthe ink to thepaper.

In accordance with the foregoing objects and first briefly describedherein, the present invention in the preferred and illustratedembodiment contemplates the employment of a resinous typel ink and aresino'us type dielectrically coated paper, both the ink and paperincluding one or more chemicalcompounds from a group ofchemical'compounds'which have a naturalafiinity for one anothery suchthat when the latent image is formed on the printing medium and themedium'is thereafter developed andmade visible'by the application ofelectrostatic ink thereto, the imagemay then be fixed'b'y thereafterpassing the paper through high pres'sureca'lendering apparatus wherebythe ink` is deformed" and fused' so as to chemically and mechanicallybond it to the recording medium with which it has an afhnity thus toyprovide a superior copy with sharper appearances, high tcolor density,permanent and durable against adverse physical conditions such asabrasion, handling, folding etc.

With these and other objects in view, the invention consistsv in a novelmethod as` will be more fully described andparticularly pointedeoutinthe appended specification and claim with reference tothe'acco'mpa'nyin'g drawings:

FIG. l is an elevational view, partially'schematic, of a preferred formof apparatus embodying; the present invention;

FIG. 2 is a viewf along the' line 2+2o'f FIG. 1 illustrating thepressure fixingassembly;

FIG. 3is a sectional viewalong the line3-3 of FIG. 1` illustratingaportion of the recording medium;

FIG. 4 is-a plan view of a greatly enlargedportion-of the recordingmedium with certain characters electrostaticallyrecorded'and-fixed-thereon according toithe present method;

FIG. 5 is agreatly enlargedview along the line S-S'fof FIG. 4;

FIG. 6` is an enlarged view of one of the spots of FIG. 4 before fixing,and?,

FIG. 7 is a similar view of the'spo'ts after fixin'gaccord# ing to ltheinvention.

Referring now to the figures of'thedawings which will bediscussedsimultaneouslyv in detail, there is seen a preferred embodimentof an electrostatic recording'apparatus 3,293,059 Patented Dec. 20, 196610, including a supply reel 12 for a supply of recording material 14,such for example, as paper tape, prepared f in accordance with theteachings hereinafter set forth. It

is to be understood of course, that although the preferred embodiment ofthe invention is directed to a tape type printer, with certainmodifications the same apparatus could be used for producing acontinuous page of any desired width of printed copy.y The presentapparatus further includes a recording transducer 16 having a backupanvil 18 adjustable by means 20 rela-tive to the head22, anelectrostatic image developing member 24, and apressure fixing mechanism26.

The web of electrostatic recording paper 14, whose chemical compositionwill be described herein later on, is affixed to the reel 12 inconventional fashion, and is fed from the reel 12 over a roller 28 on adancer arm 30 which is biased by spring 32 in a direction to applysuitable tension` to the web. Thereafter the tape is fed over a guideroller 34 thence between the head 22 and the anvil 18. The anvil 18 ismade adjustable so as to bring the paper into close proximity to thehead 22 while causing the paper to bear upon the anvil over the fullwidth and length thereof. The weby is then fed around a roller 36verticallydownwardly to a secondk roller 38 and upwardly through a slot4'0 into an ink container 42, the funnel shaped portion 44 of which isdesigned to contain a desired amount of finely particulatedelectrostatic recording ink 46, freely disposed thereon for gravity feedagainst the paper 14. Pressure guides 48 of felt, or similar material,provide a seal atA the slot 40 to prevent accidental discharge of theink into the surrounding atmosphere. The web is then fed across a beatermechanism 50 between a plurality of angled ink deflectors 52-52 to adoctor blade 54. Thereafter thepaper 14 exits from the inker mechanism24 and is fed between the rotating high pressure calendering rolls 56and 58-of pressure fixer 26.

Roller 58 of the pressure fixer 26 is mounted on one end of a lever arm60 which is pivoted Aby a pin 62 to the frame 64V of the apparatus 10;The opposite end of the lever arm 60 is biased vertically upwardly bymeans of a spring 66 and the adjustable shackle bolt 68 so that the'pressurek betweenthe rollers S6 and 58 may be varied at will lover apredetermined' range. Thus, varying degrees of calendering pressurebetween theupper and the lower calendering rollers 56 and 58 maybevprovided. From the pressure fixer 26 the printed and fixedelectrostatic image bearing' material is fed to a container (not shown)e.g., take up reel, bin 0r bucket suitably disposed adjacent the exitportion thereof for containingthe printed material until such time as itis to be used, i.e., viewed, etc.

Referring now to FIG. 5 wherein the electrostatic recording medium 14 isseen to comprise a cellulosic type base material, such for example, aspaper 70 on which there has been coated a styrenated dielectric 72 whichincludes a polyvinylacetate and a polystyrene base. This material issubstantially identical in formulation and composition to that describedin copending U.S. application to P. A.,Stowell, Serial No. 824,010, ledJune 30, 1959, now Patent No. 3,097,964, and assigned to the sameassignee as the present invention. This material has the desirablequality of remaining in colloidal suspension with many of the particlesstanding close to the surface of the coating material thus to form whatis technically referred to as tooth, i.e., a rough textured finish 74with many miniscule projections thereon and maybe produced in a mannerset forth hereinafter. The aqueous or other vehicle may be alcoholy orwater' with a fugitive base, e.g., ammonia. A polyvinylacetate copolymer(preferably a copolymer of vinyl acetate monomer with about 3% crotonicacid monomer) having carboxyl and hydroxyl groups on the polymericmolecules, the carboxyl groups being residual der the trade name Gelvafrom Shawinigan Resins Corporation, 644 Monsanto Avenue, Springfield,Mass. In an aqueous solution of ammonium hydroxide having a pH from 8.7to 9.5 there is dispersed colloidally, suflicient of such resin toproduce a dispersion having, at room temperature, the viscosity of amoderately thin syrup. For Gelva type C-3-V-20, this requires aconcentration of about twenty percent by weight of dispersion.Additional ammonium hydroxide must be added to adjust the pH tol 8.5 to8.7; it is presumed that the carboxyl groups are effective in promotingthe dispersion of the resin, and that they`cause the reduction of thepH; but this is merely a reasonable assumption. It has been foundpossible to render the final lm opaque by dispersing, as by ballmilling, in the dispersion of lyophilic resin, a pigment such astitanium dioxide sold commercially by the Titanium Pigments Company ofNew York City, New York, under the name Titanox RA 50. A weight of suchpigment equal to one-fourth the weight of the lyophilic resin suflicedto render the linal film opaque. A mechanical dispersion of a differentdielectric resin, which is insoluble and lyophobic in aqueous solutions,is prepared by ball milling in methanol for several hours a copolymerformed from styrene and alpha-methyl-styrene monomer having a meltingpoint of about 112 degrees centigrade. The ball milling is continueduntil the average particle size of the copolymer is below one micron.The concentration of the ball milled dispersion is by weight abouttwenty-live percent of resin. To the colloidal ammoniacal dispersion ofthe polyvinylacetate copolymer there is added from three to six percentby weight of the ball milled methanol dispersion of polystyrenecopolymer, mixing the two thoroughly. A polystyrenepoly-alpha-methylstyrene copolymer satisfactory for this purpose is soldby the Pennsylvania Industrial Chemical Corporation, 120 State Street,Clairton, Pennsyl- Vania, under the trade designation of Piccotex-llZ.It should be observed particularly that the polyvinylacetate copolymeris insoluble in water, and non-hygroscopic', and in accordance with acommon property of colloidal dispersions, a dispersion of this materialsuffers a rapid increase in viscosity upon a relatively small withdrawalof suspending liquid. The polystyrene (poly-alpha-methylstyrene)copolymer is reasonably hard, and suliiciently friable that it mayreadily be reduced in size by ball milling and has good dielectricproperties.

The mixed colloidal and mechanical suspension whose preparation hasalready been described, is applied to a paper base 70, FIG. 5, bypainting, rolling or dipping, or other coating techniques, with thefollowing results: the paper absorbs the aqueous ammonia solution intoits interior portion by capillary action, leaving at its surface ahighly viscous colloid gel Icontaining mechanically dispersed particlesof polystyrene poly-alpha-methylstyrene copolymer. Upon drying, the geldeposits a continuous, liexible uniform thin iilm 72 of polyvinylacetatecopolymer whose surface has what may be called tooth from the presenceof the harder particles of polystyrene poly-alphamethylstyrenecopolymer, which render the surface capable of abrading the graphite ofa pencil lead, and are believed to assist in the spreading ofconventional writing ink over the surface, as was observed to occur. Itis suspected that the hydrophilic carboxyl groups in the film render thefilm writable by uid ink.

Experimental evidence demonstrates that when the resins in thedielectric 72 are compatible with, and chemically similar to, those inthe ink 76, being used, (FIG. 6) molecular bonds can be establishedbetween the ink and the dielectric when the two are brought togetherunder applied pressure. Thus, for ideal pressure fixing the ink 76should comprise a thermoplastic resinous vehicle suitably 4 pigmentedwith carbon black so as to color the same as well as to provide adesired degree of electrical conductivity. At least a part of theresinous vehicle must include a resin which chemically is the same as orsubstantially similar to at least one of the resins which is aconstituent of the dielectric '7 2 onythe paper 70, earlier describedherein. Presuming these conditions to be met, then when sufcientpressure is applied to the ink 76, FIG. 6, to press it against thedielectric and, due to the fact that the dielectric is sufficiently hardto support the ink, enough work is done on the ink to adiabaticallyincrease its temperature to a level suicient to ca use it to softenslightly and to fuse and assume the shape of the particles 76', FIGS. 5and 7, thereby welding or bonding itself to the similar resin in thedielectric 72. An example of an inking powder possessing the foregoingqualities and properties may be formulated as follows: 500 grams ofKoppers Co. Dylex K85 is used as the binder for the ink. This materialmay be obtained from the Koppers Co., Inc. Plastics Division, 430 ParkAvenue, New York 22, New York. K85 is a styrene-butadiene latex, 50%solids in water with a styrene/butadiene ratio of 80/20. To this isadded 700 grams of Aquablak M, a dispersion of conductive furnace blackin water, 50% solids made by Columbian Carbon Co., 380 Madison Avenue,New York 17, New York, and 300 grams of water. The resultant dispersionis spray dried in a Bowen Laboratory spray dryer, 240 maximum outlettemperature. The powder is sized by screening and elutriation (thelatter as part of the drying process) to provide particles in the rangeof 16 to 100 microns.

The resinous ink particles 76 must have'adequate hardness, however, sothat a consider-able amount of work must be done to defo-rm` them and atthe same time they Imust not be excessively friable but must rather becapable of plastic deformation. In this way when high pressure isapplied to them they will collapse, spread out slightly, FIGS. 6 :and 7,and a comparative-ly large amount of work will be delivered to eachparticle with the result ythat such work will appear in the form ofenough heat to fuse the particle. Thus the frictional Work done bycompressing the relatively hard ink particles acts to dissipate heat in,and thereby raise .the temperature of, the ink particlesV before theheat can be transferred by conduction or 'otherwise to the paper coatingand to the surroundings, so that the thermoplastic ink material softensand becomes molecu'larly bonded to the chemically similar material ofIthe coating before there is yany substantial removal of thermal energyfrom the ink particles. In the event that the dielectric has a veryrough or toothed surface which is hard but there is no chemicalsimilarity between its constituent resins and the constituent resins ofthe ink particles, then the ink particles may still be fused aspredicted by the applied pressure 'and the resulting temperatureincrease but there apparently will be no molecular bonding between theink and the surface of the dielectric. The ink 76 may neverthelessappear to be fairly well adhered because of being mechanically locked bybeing pressed into the interstices and the tooth on the surf-ace of thedielectric. However, it is the combination of the mechanical bonding toa toothed dielectric surface and molecular bonding between thechemically similar and compatible resins in the pressure-fused ink andthe dielectric surface which has been found to give fixing orink-to-dielectric bonding, which is superior to either technique alone.Other examples of such chemically compatible resins include:polystyrene, polystyrene acrylonitryl, polystyrene butadiene andstyrene-alphamethylstyrene.

With the foregoing in mind it is seen that when the paper web 14 passesbeneath the recording head 22 with the recording styli, not shown, alatent image is produced upon the web which is Athereafter made Visibleby passage of the web through the recording ink 46 as hereinabove setforth, thus to produce a visible image comprising a series of dots 78which while in and lof themselves tending to be fairly irregular insize, FIG. 4, are more or less made to appear uniform-ly regular due tothe small size `of the image Irelative to the comparatively llargenumber of much smaller ink particles lon each =do t of the image and tothe fact that the eye tends to desire the regular rather than irregularin its viewing of the image, FIG. 3. As now conditioned and madevisi-ble by the application of the electrostatic recording ink to thelatent image each dot appears more or less Ias shown in FIG. 6, Iwhereinthe particles while substantially spherical are of varying size and areirregularly dispersed throughout the dot area. Thus it is seen'inrespect -to FIG. y6, that there are voids between the particlescomprising the dots as indicated by the reference character 80. Thesevoids tend to :reduce the contrast of the viewable image are relativelyundesirable. It is noted at this point 4that as is well known in theprior art, the voltages on the recording pins or sytli, generallyspeaking, control the dot size and also the separation between dots.

Thereafter as has been stated hereinabove, the web is passed between thecalendering rollers 56 .and 58, whereupon, due to the extreme 11i-ghpressure and adiabatic heating as hereinbefore mentioned, the individualink particles 76 in each of the dots 78 are heated and fused tending t-odeform and ilow together slightly to form a dot 78 of the type seen inFIG. 7, wherein the majority of the voids 80 are missing las a resultlof the fact that the adjacent continguous areas between the dots aretil-led up `due to the aforementioned flowaibility. Thus an extremelyhigh contrast image is produced having much better definition thanheretofore was obtainable with the known prior art electrostatic Xin-gtechniques.

There is thus described a method of electrostatic recording wherein arecording medium such las paper is provided with a dielectric materialwhich is sufficiently hard to cause somewhat thermoplastic recordinginkparticles to deform under calendering pressure without shattering orembedding themselves in the dielectric surface thereby enabling the inkfirmly to adhere to the recording materia-l effectively pressure weldingand molecularly bonding the lresin of the ink to the dielectric of thepaper. A

certain mechanical interlocking with the resin ink also` occurs as thepressure forces the ink to conform initimately wit-h the surfacecontours of the dielectric. This further enhances Ithe bond between theink and the dielectric.

What is claimed is:

The method of xing a latent electrostatic image Without the use ofexternally applied heat comprising the steps of:

5 (a) providing an electrostatic record medium with a dielectric thinfilm coating of styrenated resin material Iwhereby t-o prov-ide atoothed `or textured surface thereupon,

(b) preparing a particulate electrostatic recording ink with astyrenated resin material having a chanacteristic and chemicalsimilarity to the styrenated textured surface forming material in thelrecord medium, and being sufhciently softer than this material wherebythe ink particles will be deformed by the latter upon pressure contacttherewith,

(c) producing a latent electrostatic image of the desired informationupon the dielectric surface of said record medium,

(d) applying said recording ink to said dielectric surface thus toproduce a visible image of said information, and

(e) finally permanently fixing said ink to said record member by passingsaid record member between adjustable calendering rolls so as to varythe amount of the realen-dering pressure upon said record rnernberwhereby said ink is deformed and adiabatioally heated such that the inkparticles are caused to co1- lapse slightly raising their temperaturesand fusing the same substantially eliminating 'any voids therebetweenand chemically and mechanically bonding the ink to the textured surfaceof the dielectric material effectively permanently aixing the image onsaid record member.

References Cited by the Examiner UNITED STATES PATENTS 7/ 1960 Bolton346-74 12/ 1961 Epstein et al. 346-74 6/1963 Howell 117-l7.5

